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Sap flows out of trees due to the difference in pressure between the inside and outside of the tree. In the 1950s, maple researchers and producers found that adding vacuum pumps to tubing systems could increase this pressure differential and thereby increase sap yields. Use of modern equipment, materials, and techniques for tubing systems, combined with a proper vacuum pump can result in yields 2-3 times higher than that from traditional bucket (gravity) sap collection. The use of vacuum does not result in additional damage to the tree.